Conventional recording of sound and playback is performed by electronic systems of the analog type. The sound waves from a source being recorded are converted to electrical signals on a one to one basis; the acoustic sound waves have their analogy in the electrical current generated by the microphone or pre-amplifier circuit such as used in a receiver, turntable or magnetic tape source. On playback the electrical current is amplified and used to drive loudspeakers which convert the electrical signal to sound by the mechanical motion of an electromagnet and speaker cone.
As a consequence, the output of conventional recording and playback systems consists of waveforms either cut into a vinyl medium or imposed on magnetic particles on tape. The accuracy of the reproduced wave form is directly dependent on the quality of the metal or plastic disk or of the tape itself. Both the production of disk copies and tapes and their means of playback tend to degrade the quality of the reproduced analog signal. Noise, in the form of contamination, wear and the inherent background output of the medium itself is therefore unavoidably present in the recording and playback systems utilizing conventional analog recording and playback technology. Recent developments in audio-digital sound recording and playback systems represent efforts to reduce or eliminate this noise problem. Exemplary of such developments are the kinds of systems and equipment disclosed in the following patents: Meyers et al, U.S. Pat. No. 3,786,201 issued Jan. 15, 1974; Borne et al, U.S. Pat. No. 4,075,665, issued Feb. 21, 1978; Yamamoto, U.S. Pat. No. 4,141,039, issued Feb. 20, 1979; Stockham, Jr. et al, U.S. Pat. No. 4,328,580 issued May 4, 1982; Tsuchiya et al, U.S. Pat. No. 4,348,699 issued Sept. 7, 1982; and Baldwin, U.S. Pat. No. 4,352,129 issued Sept. 28, 1982, the disclosures of which are specifically incorporated herein by reference. These systems are characterized generally as taking advantage of the high speed operation of the digital electronic computers. The signal waveform, representative of sound in such digital sound recording and playback systems, is frequently sampled to produce a serial stream of data that is translated into a binary code that assigns a numerical value for each sample. This can be visualized as slicing up a continuous curve into a large number of very short step-like segments. The process is reversed on playback as each numerical value of each segment is converted into an output voltage. When this process is done rapidly enough the fact that the sound wave has been "chopped up" and re-assembled cannot be detected by the human ear. When sound is recorded in digitized binary code in this manner, the sound, such as music, is only a series of numbers represented by magnetic particles which, when read by the appropriate electronic means, are either "on" or "off" with no intermediate values. Such binary signals are virtually immune to distortion, error, and degradation with time. All sources of noise normally associated with analog devices are eliminated, that is, there is no tape hiss, no tracking errors, no surface effects. Signal to noise ratios are obtained only by the digital to analog conversion circuit itself and the power amplifiers.
These systems do, however, have several drawbacks. A representative system currently in use for recording master tapes in the record industry has excellent audio qualities as a result of a high speed sampling rate of 50 KHz and good digital binary code resolution in the form of a 16 bit word for each sample. The problem with this system is that every sample must be preserved in mass storage for playback. The storage system thus must hold on the order of 4,320,000,000 bits of information for a 45 minute record. Storage systems of this capacity are large, expensive, and generally not suitable for a consumer product.
Attempts to resolve the storage capacity problem have taken the approach of reducing the resolution of each sample (fewer bits per "word") while at the same time reducing the sampling rate (to 12 khz). Such reductions have reduced the data storage requirement by as much as a factor of 4. The resulting fidelity of the output, however, is often below that acceptable for high fidelity sound recordings of music.
Another approach much favored by telephone companies, employs the foregoing reduction of bits described above and in addition adds the restriction of input signal band width to that most used by talking voices (3 to 8 KHz). A total data reduction factor of about 12 is possible in this manner, again accompanied with a reduction in sound quality.
Recent attempts at a solution to the storage problem and the fidelity reduction problem utilizes ultra high density digital storage by laser recording technology. This has been partially successful in that adequate playing times have been achieved with the improved storage capacity. However, the manufacturing technology and equipment presently necessary to create a "laser-burned hole", "pit", or "black spot" in the storage medium restricts "laser disks" or "laser fiches" to the "playback only" mode with no potential for in-home recording or erasing and editing.
It is therefore an objective of the present invention to provide a system for high fidelity sound recording and playback that does not have the foregoing drawbacks and associated problems.